The provision of ventilation outlets in order to vent a secondary flow of fluid into a primary flow of fluid are known in a wide range of different fields. For example, it is known to provide a ventilation outlet as part of a gas turbine engine, in order to vent a stream of hot gas from the so-called “fire zone” or core of the engine into a main gas stream, such as a relatively cool bypass flow passing through a bypass duct extending around the engine shroud.
FIG. 1 illustrates a simple vent outlet of a type proposed previously. As can be seen, the vent outlet 1, which is provided at the end of a ventilation duct, is formed flush with the surface of an engine casing 2, and may comprise one or more louvers 3 extending across the outlet. The hot stream of vent gases is indicated by arrow 4, and this is directed through the vent outlet and into a relatively cool bypass flow indicated by arrow 5, and is thus ejected from the ventilation duct into the bypass flow 5. However, a problem with this arrangement is that it is not particularly effective at mixing the hot flow of vent gas with the cool bypass flow, with the result that the hot gas impinges on the downstream surface of the engine casing and other components in that region. This leads to a “hot streak” on the engine casing and can cause significant thermal damage to the structure unless it is properly protected from the heat, which can increase the weight of the engine as well as the overall cost.
Similar problems can occur with conventional bleed valve arrangements in gas turbine engines, which are usually used to improve engine operability. In use, the heated air at high pressure passes from a compressor, through a bleed valve and via a diffuser into a main gas stream, such as the relatively cool bypass flow. The bleed valve allows this bleed flow to be actively or passively managed in sympathy with the operating characteristics of the engine at any particular instant in time. The diffuser, which typically takes the form of a so-called “pepperpot”, is used partly to attenuate noise produced within the bleed valve itself, but also in some arrangements to produce turbulence in the flow for the purposes of enhancing mixture of the hot bleed flow with the cool bypass flow, thereby at least partly addressing the above-mentioned problems arising from the hot gases impinging on downstream parts of the engine shroud and other components. However, pepperpot diffusers are typically relatively large and heavy components, and their use can thus be expensive.